Very well said, Steve. I think the key point you make is : "Bounce observable on the PCB is a PCB problem not a package problem". /dan --- steve weir <weirsp@xxxxxxxxxx> wrote: > Aubrey, Mike, > > In response to that EET article, I sent the > following letter to Mr. > Goering. The "Reader's Digest" version is that: > > 1. The PCB can kill the system just as easily as the > IC. Both are part of > the package. > 2. The OEM is responsible for obtaining enough data > to design a system, as > opposed to creating a netlist and praying. > 3. Even where documentation is lacking, > characterization can yield > sufficient data to design reliably. > 4. Bounce observable on the PCB is a PCB problem not > a package problem. > 5. There are lots of misconceptions out there about > power distribution and > packaging. > 6. Package limitations in the ICs, at the board > level and in the system are > a fact of life. Making it all work is what we are > paid to do. > 7. This year's DesignCon has about a dozen papers ( > shameless > plug: including two from Teraspeed ) and > presentations on topics that > touch on the subject matter of the Dec. 18 EET > article. > > I hope everyone has a happy and safe holiday. > > Best regards to all, > > > Steve. > > ==== > (c) 2004 Teraspeed Consulting Group, LLC > > Dear Mr. Goering, > > I read with great interest, Monday's piece: "When > bad packages kill good > pc boards". I hope someday you will run a > complementary piece perhaps > titled "When > bad board designs undermine serviceable packages". > > To blame excessive system noise on the IC package > without examining how the > actual noise budget was developed and verified, I > find a bit like blaming only > the girl for an unwanted pregnancy. It is the OEM's > ultimate responsibility to > engineer their system properly. This is a theme that > Lee Ritchey stumps on in > his book, "Right the First Time". In his excellent > DesignCon 2004 paper > "System Level > Power Integrity Analysis and Correlation for > Multi-gigabit Designs" Dr. > Ralf Schmitt > and his fellow Rambus authors made plain that system > level power integrity > is the > sum of the parts from the IC core all the way out > through the PCB structure to > the voltage regulator module. If a system is built > without the requisite > analysis, then why > is anyone surprised if it does not work properly? > > OEMs who wish to be successful must develop the > internal skills, or import > external expertise to engineer their systems by > design, and not happenstance. I > find little surprise that various FPGA customers who > have spent the > resources to > develop such expertise jealously guard it, and do > not wish to share it with > competitors. > > Similarly, why would anyone expect an application > note to circumvent the need > for engineering? An appnote is a place to get > started, not the end-point. A > good > appnote IMO demonstrates a principle and provides a > distilled example. Why is > such cookbook simplification reviled instead of > acknowledged for the > limitations > it necessarily carries? As with the hilarious > Douglas Adams books, the trouble > isn't understanding the answer, it is understanding > the question. > > If an OEM lacks the internal resources to evaluate > feasibility, and then design > a demanding physical system themselves, that > critical task should be > out-sourced > to appropriate expertise. That expertise is > available. In the meantime, > education is available including from colleges, > consultants, and even vendors > such as Xilinx. To wit for a paltry $20. one can > purchase the DVD set just > released featuring Dr. Howard Johnson's discussion > of the care and feeding of > MGTs. Xilinx also offers an SI course, and Altera > will be presenting at > DesignCon 2005 on SSO. > > We have many people in this community, including the > many contributors to the > SI-List from Sun Microsystems: Larry Smith, Istvan > Novak, Ray Anderson ( > recently snagged by Xilinx ), who for years have all > been beating the drum on > the importance of three things to PDS quality: > Inductance, inductance and > inductance. Even so, misconceptions, and failure to > do necessary homework > frequently get people into trouble. One subject that > I still see regularly > trip-up even "experts" is a proper accounting of > device via attachment and > plane > spreading inductance as elements of the overall > bypass impedance budget. > Overlooking these in high performance systems can > lead to horribly wrong > conclusions and encourage error prone measurements. > Yet we see repeated > statements and even books that assert demonstrably > wrong claims in both > theory and practice. Given the description offered > in today's article that > featured Mahi's difficulties I can but wonder if > this common > misunderstanding is > not at the root of the nine-month $20 million dollar > fiasco reported. > > The article quotes as an example a reported failure > at Mahi Networks, described > as: > > "Mahi was designing what Hecker describes as a > "cutting-edge application" > involving 3.125-GHz backplane links. The design > called for FPGAs with > integrated > serdes capability to be used as serial links. But > test data showed severe > ground > bounce and power bounce on the board, as well as > performance problems > related to > the operation of the serdes links in the system > environment." > > Assuming the quote is accurate, how on earth does > any engineer blame board > level > bounce on the IC package? Were the package the > culprit here, noise at IC > package > attachment would be nominal while the noise at quiet > IC I/Os or in the core > would be problematic. > > For board level PDS stability, the only two > variables at our disposal are > the IC > application current and the board impedance. The > former may be determined by > test fixturing, the latter readily synthesized by > design. So how could anyone > who did their engineering homework end-up with > unexpected excessive board level > bounce? How could such bounce possibly be the fault > of the package as > opposed to > the system engineer? Placing a high performance IC > in an inadequate PCB > environment is a lot like putting Yugo tires on a > Formula One race car. Why is > the tragic result either a surprise, or blamed on > the car? > > There are many misconceptions out there, and none of > them help engineer working > systems. Unfortunately, Monday's piece IMO helped > propagate some wive's > tales. I > about fell out of my chair when I saw that article > apparently === message truncated === __________________________________ Do you Yahoo!? Jazz up your holiday email with celebrity designs. Learn more. http://celebrity.mail.yahoo.com ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu